Liquefied gases, such as liquefied natural gas (LNG, boiling point: −164° C.), liquefied oxygen (boiling point: −183° C.), and liquefied nitrogen (boiling point: −196° C.), require ultra-low temperature storage conditions. Thus, a structure, such as a pressure vessel formed of a material having sufficient toughness and strength at an ultra-low temperature is required to store these gases.
A chromium (Cr)-nickel (Ni)-based stainless steel alloy such as AISI304, a 9% Ni steel, and 5000 series aluminum alloys have been typically used as materials usable at the low temperatures of liquefied gas environments. However, with respect to the 5000 series aluminum alloys, the use thereof may be limited, because alloy material costs may be high, a design thickness of a structure may be increased due to the low strength of such alloys, and welding workability may be poor. Cr—Ni-based stainless steel and 9% Ni steel largely overcome the limitations in terms of the physical properties of aluminum. However, there have been limitations in the use thereof, for example, due to manufacturing costs being increased due to the addition of relatively expensive nickel.
In order to address these limitations, Patent Documents 1 and 2 disclose techniques of decreasing an amount of relatively expensive nickel and adding manganese and chromium instead. Patent Document 1 discloses a technique of improving ultra-low temperature toughness by securing an austenitic structure by decreasing the amount of nickel to 1.5% to 4% and adding 16% to 22% of manganese and 2% to 5.5% of chromium instead. Patent Document 2 discloses a technique of securing ultra-low temperature toughness by refining ferrite grains through repeated heat treatments and tempering while decreasing the amount of nickel to about 5.5% and adding 2.0% of manganese and 1.5% or less of chromium instead. However, since the above inventions also still contain relatively expensive nickel and various stages of repeated heat treatments and tempering are required to secure ultra-low temperature toughness, it may not be advantageous in terms of cost or process simplification.
Another technique related to a structural steel used in forming liquefied gas containment environments may include a so-called “Ni-free high manganese steel” from which nickel is completely excluded. The high manganese steel may be divided into ferritic and austenitic steels according to the amount of manganese added. For example, Patent Document 3 discloses a technique of improving ultra-low temperature toughness by adding 5% of manganese instead of 9% of nickel to refine grains through four heat treatments in a two-phase temperature range in which austenite and ferrite coexist, and then tempering. Also, Patent Document 4 discloses a technique of improving ultra-low temperature toughness by adding 13% of manganese to refine grains through four heat treatments in a two-phase temperature range of austenite and ferrite, and then tempering. The above patents include ferrite as a main structure, and have a main characteristic in which ferritic grains are refined through four or more heat treatments and tempering to obtain ultra-low temperature toughness. However, these techniques may have limitations in that costs may increase and heat treatment equipment may be overloaded due to an increase in the number of heat treatments.
Patent Document 5 discloses a technique related to a high manganese steel having excellent ultra-low temperature characteristics, in which a large amount, i.e., 16% to 35% of manganese and 0.1% to 0.5% of carbon are added instead of completely excluding nickel to stabilize austenite and 1% to 8% of aluminum is added. Patent Document 6 discloses that a high manganese steel having excellent low-temperature toughness may be obtained by forming a mixed structure of austenite and ε-martensite through the addition of 15% to 40% of manganese. However, since the amount of carbon is low, toughness may deteriorate due to the generation of a structure that is unstable at ultra-low temperatures, such as a ε-martensite structure. Also, the possibility of the occurrence of casting defects may increase due to the addition of aluminum.
Furthermore, since an austenitic high manganese steel may have poor machinability due to high work hardening, the lifespans of cutting tools may be decreased. Accordingly, production costs, such as tool costs and down times, related to the replacement of tools, may be increased.